Membrane filtration methods and membrane filtration device

Reverse osmosis membrane to obtain a pretreatment membrane module to obtain a pre-treated water was filtered with a filtration membrane raw water, the pretreated water obtained by the pretreatment membrane module by filtration through a reverse osmosis membrane with permeate water and concentrated water module and a communication duct for coupling the pre-treated water inlet of the pre-treated water outlet of the pretreatment membrane module and said reverse osmosis membrane module directly, the pretreated water flowing the communicating conduit some of the have a branch pipe branched from the communicating pipe deriving from the communicating pipe, by using the pre-treated water that has been derived by the branch pipe, the filtration membrane of the pretreatment membrane module membrane filtration method for performing backwashing and membrane filtration units.